Most DV cameras allow you to record using one of two audio settings: 12-bit or 16-bit audio. If you used both of these settings on one tape (some of the footage was shot with 12-bit audio, and some was shot with 16-bit) and then use iDVD 5 to create a OneStep DVD project, iDVD may have difficulty keeping the audio synchronized with the video.
To avoid this issue, always keep your camera set to 16-bit audio, if possible. This will ensure higher quality audio and better compatibility with iLife applications.